This invention relates to a method of controlling the fracture orientation of hydraulic fractures in underground formations to increase well productivity.
Hydraulic fracturing is a well established method used in the oil and gas industry for reservoir stimulation. The general technique is to inject fluid under high pressure into a well-bore and perforated formation to create fractures in the hydrocarbon bearing formation. It was first applied in the oil industry in 1948 to stimulate productivity from low permeability oil bearing formations.
A problem frequently encountered with hydraulic fracturing is that the fracture orientation is not optimal for maximum well productivity. The orientation of a fracture in an underground formation is generally controlled by the in-situ stress of the formation. The formation is subjected to three principal stresses, one vertical and two horizontal. When a formation is hydraulically fractured the created fracture should propagate in the path of least resistance or, in other words, the fracture will be perpendicular to the least principal stress.
In deeper formations (generally below 2000 ft.), one of the horizontal stresses is usually the smallest stress because of the high weight of the rock. Consequently, a vertical fracture is created. The above is also generally true for any natural fracturing which may be present in the formation. It is a common experience that augmenting either natural or man-made hydraulic fractures with further hydraulic fracturing results in parallel fractures which do not significantly increase the productivity of the well.
Warpinski et al. (SPE 17533, SPE Rocky Mountain Regional Meeting, Casper, Wyo., May 11-13, 1988) suggests that the technique of altered stress fracturing may be used to overcome the problem of hydraulic fracturing paralleling permeable natural fractures. Warpinski et al. discusses the concept of using an offset well to create hydraulic fractures that alter a stress field around a production well. It states that if the stress difference is not too large, the wells are relatively close together and the treatment pressures and fracture sizes in the offset wells are sufficiently large, enough stress can be added to the virgin minimum horizontal in situ stress to make it the maximum horizontal stress. Warpinski speculates that a possible application of the stress alteration concept is for the alteration of the vertical distribution of the minimum horizontal in-situ stress in a single vertical hole. This could be used to advantage if hydraulic fractures are propagating into undesirable zones.
U.S. Pat. No. 4,724,905 discloses the use of hydraulic fracturing in one well to control the direction of propagation of a second hydraulic fracture in a second well located nearby. The first well is fractured and the fractures will generally form parallel to the fractures in the natural fracture system. The hydraulic pressure is maintained in the first well and another hydraulic fracturing operation is conducted at a second well within the zone of in-situ stress alteration caused by the first hydraulic fracture. This patent states that the second hydraulic fracture initiates at an angle, often perpendicular, to the first hydraulic fracture.
U.S. Pat. No. 4,830,106 discloses the use of simultaneous hydraulic fracturing in at least two spaced apart wells to control the direction of propagation of the fractures. This simultaneous pressure causes the fractures to curve away from each well or towards each well depending on the relative position and spacing of the wells in this stress field and the magnitude of the applied far field stresses. These generated fractures may then intercept at least one natural hydrocarbon bearing fracture.
U.S. Pat. No. 4,834,181 discloses the alteration of in-situ stress conditions using sequential hydraulic fracturing. The well formation is hydraulically fractured causing at least one vertical fracture to form. Thereafter a plugging material is directed into the created fracture and the material is allowed to solidify. A second hydraulic fracture is formed which should divert around the plugged fracture. The steps of plugging, hydraulically fracturing and diverting the subsequently created fracture are continued until branched or dendritic fractures are caused to emanate into the formation from the wellbore. U.S. Pat. No. 4,687,061 teaches the simultaneous fracturing of a borehole at two different levels in a deviated well.
None of the above methods are totally satisfactory. The methods using two wells are complex and hard to control. Additionally, these methods typically are not practical in fields with well spacing requirements. In the method disclosed in U.S. Pat. No. 4,834,181the direction of the sequential fracturing is not controlled from the wellbore and it is merely a matter of chance as to whether the branch fractures will run perpendicular to either the natural fractures in the formation or the earlier induced hydraulic fractures. U.S. Pat. No. 4,687,061 does not disclose a method to control the direction of the propagation of the fracture from the wellbore, nor does it disclose using the method in a vertical hole. The industry is still in need of a method which can with some predictability control the orientation of hydraulic fracturing from a single wellbore.